FIG. 13A shows a highly simplified form of a conventional painting system 1 for painting vehicle body parts, wherein the painting system 1 comprises as application device a rotary atomizer 2 with a bell cup 3, which produces a spray jet 4 of paint while operating. The rotary atomizer 2 is hereby supplied with the paint to be applied over a color line 5, a main needle valve 6 being located in the color line 5 which guides the flow of paint to the rotary atomizer 2 and which is opened during a coating process. Furthermore, the painting system 1 is fitted with a return line 7 in which a return valve 8 is located, wherein the return line 7 opens into a return system in order to capture residues remaining during a color change (for example flushing agent, residual color, color foam) and to dispose them economically.
When changing the color in the painting system 1, firstly the main needle valve 6 is closed whereby the flow of paint to the rotary atomizer 2 is interrupted. The return valve 8 is then opened and the color line 5 is cleaned using a flushing agent and compressed air (blasts of compressed air), the residues of the paint remaining in the color line 5 being led over the opened return valve 8 and through the return line 7 into the return system. Subsequently, the color line 5 is loaded (pressurized) with the new paint of the desired color, wherein the main needle valve 6 is still closed while the return valve 8 is opened. The loading can be stopped when no more color foam or compressed air comes out in the return line 7 downstream behind the return valve 8 but fresh color. This is recognized by the painting system 1, in this example by a light barrier which consists of a source of light 9 and an optical sensor 10, wherein the light source 9 and the sensor 10 are located on opposite sides of the in this area transparent return line 7 so that the light barrier detects the transparency to light of the fluid coming out behind the return valve 8. During flushing the color line 5, a mixture of flushing agent, residual color and compressed air flows through the return line 7, this mixture being relatively transparent to light due to the high proportion of air in it. After loading of the color line 5, fresh paint appears at the outlet of the return valve 8 which contains hardly compressed air and is therefore relatively impervious to light. The sensor 10 is connected on the output side with a control unit 11 which closes the return valve 8 if the light barrier detects that fresh paint appears at the outlet of the return valve 8.
FIG. 13B shows a modification of the painting system 1 according to FIG. 13A which is different in that the return line 7 here branches off from the color line 5 downstream behind the main needle valve 6.
A disadvantage of the painting system 1 described above is firstly the fact that the return line 7 can get dirty during operation so that the light barrier can no longer detect the transparency to light of the flowing through fluid.
A further disadvantage of the conventional painting system 1 described above is the reaction time between reaction of the optical sensor 10 and closing the return valve 8, wherein fresh paint is led away over the return line 7 during the reaction time which leads to respective losses of color.
The conventional painting system 1 described above also needs additional components in the form of the source of light 9 and the optical sensors 10 to control the return valve 8, whereby the costs for manufacture and commissioning and also the fault liability increase.
Furthermore, it is also known from the prior art that return valve 8 can be actuated during a color change without a light barrier according to a prescribed time program. Here, the assumption is made that during a color change after a flushing operation, the fresh paint appears at a particular time in the return line 7 downstream behind the return valve 8.
A disadvantage of using this known control for the return valve 8 based on a time program is the fact that the time required during a color change for flushing and subsequent loading of the fresh paint depends on the viscosity of the paint so that a wrongly adjusted viscosity or fluctuations in the viscosity lead to inaccurate controlling of the return valve 8. If the return valve 8 is closed too late then fresh paint will be led over the return line 7 into the return system which is associated with corresponding losses of color. If, on the other hand, the return valve 8 is closed too early there will still be residues of the prior flushing operation in the color line 5 upstream before the return line 8 whereby the painting quality would be impaired after opening the main needle valve 6.
Accordingly, there is a need to appropriately improve on the conventional painting system 1 described above, in particular, with the option being provided to control the return valve 8 with the least possible effort and as precisely as possible when color is changed.